Optical recording media, such as a CD with a storage capacity of 0.65 GB and a DVD with a storage capacity of 4.7 GB are becoming pervasive as means for saving video information, audio information, or computer data. Furthermore, in recent years and continuing, there are increasing demands for higher recording density and larger storage capacity.
In order to increase the recording density of such an optical recording medium, it is effective to reduce the diameter of a beam spot formed with an optical pickup configured to write/read information in/from the optical recording medium. The beam spot is formed on the optical recording medium by condensing a light beam with an object lens. The diameter of the beam spot can be reduced by increasing the numerical aperture (hereinafter, also abbreviated as “NA”) of the object lens, or by shortening the wavelength of the light beam emitted from a light source.
For example, the NA of the object lens is 0.50 and the light source emits 780 nm wavelength light for a “CD-system optical recording medium”, while the NA of the object lens is 0.65 and the light source emits 660 nm wavelength light for a “DVD-system optical recording medium” with which a higher recording density is achievable than the “CD-system optical recording medium”. As described above, there are increasing demands for higher recording density and larger storage capacity in optical recording media. Thus, it is desirable to make the NA of the object lens greater than 0.65 and to make the wavelength of light emitted from the light source shorter than 660 nm.
Two standards have been proposed for such large-capacity optical recording media and optical information processing devices. One is the standard of “Blu-ray Disc” (hereinafter, “BD”), which uses a light source of a blue wavelength region and an object lens with an NA of 0.85 for securing a storage capacity of 22 GB. Another is the standard of “HD-DVD” (hereinafter, “HD”), which also uses a light source of a blue wavelength region, but uses an object lens with an NA of 0.65 for securing a storage capacity of 20 GB.
In the BD standard, the storage capacity is increased by making the wavelength shorter and making the NA larger than those of the HD standard. In the HD standard, the storage capacity is increased not by enlarging the NA but by improving the track recording density by making adjustments in signal processing, and by performing a land/groove recording method.
BD and HD both use a violet semiconductor laser light source with an oscillation wavelength of approximately 405 nm. However, BD and HD employ optical recording media having different substrate thicknesses, i.e., 0.1 mm and 0.6 mm, respectively.
In an optical pickup for recording and/or reproducing high density information based on BD and HD, the optical system is preferably applicable for both BD and HD in case the standards of BD and HD become equally pervasive.
Accordingly, such an optical pickup desirably has the following configuration. Specifically, a light source of an appropriate wavelength is selected according to the type of the optical recording medium to be used for recording or reproducing. An appropriate optical process is performed on a light beam emitted from the selected light source, in order to correct the difference in spherical aberration caused by the difference in the substrate thicknesses of the optical recording media.
In order to record or reproduce information of two different types of optical recording media with the use of a single optical pickup, there have been proposed means for using a single object lens and a phase difference adjusting surface (see patent document 1).
Furthermore, the inventors of the present invention have proposed, in Japanese Patent Application No. 2007-006975, means including an object lens and a diffraction surface to form a twin spot.
Patent Document 1: Japanese Patent No. 3613745
However, the invention described in patent document 1 is for correcting the aberration, which is caused by the difference in the substrate thicknesses, by applying a phase difference only for a region corresponding to a single ring-like range. This is because it is designed to be compatible for both DVD and CD having lower recording densities than those of the BD and HD standards. As described in patent document 1, when an object lens for DVDs and a phase difference adjusting element are provided before a transparent substrate of a CD, the shape of the wavefront aberration appears to be as illustrated in FIG. 30. That is, the aberration is large only around the radius position corresponding to the numerical aperture of the CD, compared to the phase of the wavefront near the optical axis. Accordingly, it is only necessary to provide a region forming a phase difference around the numerical aperture of the CD.
However, when an object lens with a high NA of approximately 0.85 is used in combination with short wavelength light to correct the aberration caused by the difference in thickness of the substrates, the wavefront aberration is extremely large as shown in FIG. 31. Therefore, although the method of applying a phase difference only for a single ring-like range is effective for correcting the aberration in the case of the DVD-system and the CD-system, the same method is insufficient for correcting the aberration in the case as illustrated in FIG. 31.
Furthermore, in the invention described in Japanese Patent Application No. 2007-006975, a twin spot is formed with the use of zero-order diffracted light and first-order diffracted light. Therefore, the diffraction efficiency of each light is approximately 40% at maximum, which leads to a decrease in the light utilization efficiency.